1. Field of the Invention
The present invention relates to an apparatus and method for driving a light-emitting device, and, more particularly, to a technology for controlling the luminance of light emitted from a charge storage light-emitting device.
2. Description of the Related Art
An organic electroluminescence (hereinafter referred to as organic EL or simply EL) device as a charge storage light-emitting device emits light by permitting current to flow through a phosphor (organic EL layer) formed on a glass plate, as a transparent substrate, or a transparent organic film. A variety of display apparatuses using such organic EL devices have been proposed.
Organic EL devices which can emit lights independently pixel by pixel are arranged on an image display. Each of the organic EL devices generally has the same structure where an ITO (anode) layer, a light-emitting layer (organic EL layer) and a cathode are deposited in order on a transparent substrate. They are also common in emitting light with an instantaneous luminance proportional to the drive current.
While a scheme called simple matrix driving is known as one method of driving an organic EL device, various other schemes involving active matrix driving have also been proposed.
The active matrix driving, which is accomplished by using TFTs (Thin Film Transistors), can provide EL devices with an excellent memory property (emission keeping property) which could not be achieved by the simple matrix driving.
More specifically, according to the active matrix driving, the drive current is supplied to an EL device from a drive voltage source through a TFT of which switching action permits ON/OFF of light emission. Weighting of the luminance of emitted light for gray scale displaying is done by amplitude modulation or time modulation (so-called sub-fielding).
The amplitude modulation is a scheme to adjust the instantaneous luminance of an EL device by controlling the drive voltage (drive current) with a constant emission time. That is, the amplitude modulation is based on an idea of controlling the intensity of emitted light to provide a desired luminance level.
The time modulation is a scheme of controlling the emission time in each predetermined period (one field period) with the instantaneous luminance of an EL device which is a constant. That is, the time modulation is based on an idea of acquiring the apparent luminance by controlling the emission rate to provide a desired luminance level.
For the time modulation, which requires a constant instantaneous luminance all the time, a constant voltage power source is normally used for the drive voltage source for an EL device.
However, the drive voltagexe2x80x94drive current characteristic of an organic EL device varies depending on the ambient temperature as shown in FIG. 1. A temperature change leads to a variation in drive current, thereby changing the instantaneous luminance. With the same voltage applied to an organic EL device, therefore, the intensity of emitted light increases at a certain temperature but decreases at a different temperature.
Since the variation in instantaneous luminance impairs the linearity of a gray scale, an accurate gray-scale image is not always displayed on an image display using the EL devices.
Accordingly, it is an object of the present invention to provide a drive apparatus and drive method for a charge storage light-emitting device, which can keep the luminance of emitted light at a constant level even when an operational temperature varies.
To achieve this object, a drive apparatus according to one aspect of the present invention comprises drive-voltage applying means for applying a drive voltage to the charge storage light-emitting device; drive-current limiting means for limiting a drive current to be supplied to the charge storage light-emitting device; voltage detection means for detecting a terminal voltage between both electrode terminals of the charge storage light-emitting device; and voltage control means for controlling a value of the drive voltage in accordance with a result of detection performed by the voltage detection means.
In the drive apparatus, the voltage detection means may be designed to detect the terminal voltage in a state of blocking supply of the drive current to the charge storage light-emitting device after the drive current has been supplied to the charge storage light-emitting device by applying the drive voltage to the charge storage light-emitting device.
In any of the modes of the drive apparatus, the voltage control means may be designed to control the drive voltage in such a way that a voltage value acquired by subtracting the terminal voltage from the drive voltage becomes a predetermined value.
In any of the modes of the drive apparatus, the drive-current limiting means may be comprised of a switching transistor.
To achieve the above object, according to another aspect of the present invention, a drive apparatus for a charge storage light-emitting device for applying a drive voltage to the charge storage light-emitting device to supply a drive current thereto, thereby causing the charge storage light-emitting device to emit light, comprises spatial-charge-voltage detection means for detecting a spatial charge voltage of the charge storage light-emitting device.
The drive apparatus may further comprise voltage control means for controlling the drive voltage in such a way that a voltage value acquired by subtracting the spatial charge voltage from the drive voltage becomes a predetermined value.
In the drive apparatus, an organic EL device may be used as the charge storage light-emitting device.
To achieve the above object, according to a further aspect of the present invention, a drive method for a charge storage light-emitting device for applying a drive voltage to the charge storage light-emitting device to supply a drive current thereto, thereby causing the charge storage light-emitting device to emit light, comprises the steps of, after supplying the drive current to the charge storage light-emitting device by applying the drive voltage thereto, blocking supply of the drive current to the charge storage light-emitting device with the drive voltage being still applied thereto; detecting a terminal voltage between both electrode terminals of the charge storage light-emitting device immediately after the supply of the drive current is blocked; and controlling the drive voltage in such a way that a voltage value acquired by subtracting a value of the terminal voltage from a value of the drive voltage becomes a predetermined value.
To achieve the object, according to a still further aspect of the present invention, a drive method for a charge storage light-emitting device for applying a drive voltage to the charge storage light-emitting device to supply a drive current thereto, thereby causing the charge storage light-emitting device to emit light, comprises the steps of detecting a spatial charge voltage of the charge storage light-emitting device; and controlling the drive voltage in such a way that a voltage value acquired by subtracting a value of the spatial charge voltage from a value of the drive voltage becomes a predetermined value.
In any mode of the drive methods, an organic EL device may be used as the charge storage light-emitting device.